An ethanol plant typically produces ethanol by fermentation of sugars using yeast. The sugars can be derived from a variety of raw plant materials. In particular, the sugars are commonly derived from sugar-containing plant materials such as sugar cane or beets, from the conversion of starch obtained from corn, other grains, or any other starch containing materials, or from the conversion of cellulosic materials.
In a typical prior art process, the raw plant materials are usually prepared in various pre-treatment steps that free the sugars. In pretreatment, water and/or backset is added to prepare the raw materials for conversion to sugar and subsequently from sugar to alcohol (i.e., ethanol) in fermentation. Backset refers to liquids, with or without solids, that are produced as a result of subsequent process steps, and are then recycled back for use in earlier process steps.
Once the sugars have been freed by pretreatment, the sugars are fermented to ethanol, and the resultant mixture of ethanol, solids, and other dissolved substances such as acids, oils, salts, and proteins, is termed beer. The beer is separated by distillation into ethanol and whole stillage, which contains a mixture of liquid, solids, solubles, and other dissolved substances. The whole stillage may then be thickened by evaporation prior to separating the liquids, solubles, and fine particles from the solids. Separation is typically done by means of a decanter centrifuge that separates the whole stillage into thin stillage (centrate) and Wet Distillers Grains (WDG). The thin stillage usually is thickened by evaporation and separated into evaporator condensate and syrup, a high-solids liquid product. The WDG can be sold as is or recombined with the syrup and sold. Most commonly, the WDG is dried, alone or along with all or a portion of the syrup, to produce Dried Distillers Grains (DDG) or Dried Distillers Grains with Solubles (DDGS), respectively. DDG and DDGS can be sold and shipped world-wide because they have an extended shelf life. DDG and DDGS are typically used in animal feed. A portion of the thin stillage is recycled into the ethanol process as backset, i.e., a portion of the thin stillage is used to offset some of the water needs of the process.